1. Field of the Invention
The invention relates to a method for determining connection quality in a receiver of a cellular radio system. In the method, a received signal is decoded with a Viterbi decoder whose output provides, in addition to the decoded symbols, estimates of the merit of the decisions made.
The invention also relates to a receiver comprising means for decoding a received signal in accordance with the Viterbi algorithm. The means calculates estimates of the merit of decisions made in addition to decoding the received symbols.
2. Description of Related Art
In a cellular radio system, the quality of the connection between a base station and a subscriber terminal varies continuously. This variation is due to disturbances in the radio path and to the attenuation of radio waves as a function of distance. For example, when a terminal moves further away from the base station, attenuation over the connection between the terminal and the base station increases. It is often attempted to compensate for this attenuation by gain control.
Gain measurement as such is not a sufficient parameter for estimating connection quality. A unit universally employed to express connection quality in digital cellular radio systems is the bit error rate (BER), representing the number of incorrectly received bits compared to all received bits. In the GSM cellular radio system, for example, the base station and the terminal are continuously measuring the bit error rate at the air interface. The results of the measurement are transferred to a base station controller that makes a decision, according to the measurement result, on a handover to a channel offering better connection quality, if necessary.
The bit error rate measurement must be as accurate as possible to enable optimum utilization of the resources of the cellular radio system and to minimize the number of poor connections. In the GSM system, for instance, the performance of the bit error rate measurement has been specified in the GSM 05.08 specification, which is incorporated herein by reference. In accordance with this specification, the measurement period for the bit error rate is 480 milliseconds, which corresponds to 104 TDMA time slots with full rate channels and 52 TDMA time slots with half rate channels.
Estimating the channel bit error rate by `pseudober` measurement is previously known. In this method, the bit error rate is estimated by encoding the channel decoding result again and calculating the number of corrected bits. This is not the best possible method, however, since there is no guarantee that the channel decoder always corrects an erroneous bit. The channel decoder may also change a correct bit into an incorrect bit. In addition, the accuracy of the method is very much dependent on the length of the bit sequence being treated. The bit sequence, i.e. the measurement period, should be long enough to enable correct detection of the bit error rate also when erroneous bits occur seldomly in the channel. In GSM, this will present a problem particularly in half rate channels wherein the number of bits is smaller than in full rate channels.
Another known method for estimating the bit error rate is to calculate the bit error rate from the known training sequence in each traffic channel during the measurement period. Since in the GSM system there are 26 bits in the training sequence and over a half rate channel there are 52 frames during a measurement period in each traffic channel, there are only 52*26 known bits wherefrom the bit error rate could be calculated. This is too few to be able to produce a sufficiently accurate bit error rate estimate.
Utilizing Viterbi decoder soft decision outputs to improve the signal-to-noise ratio of the received signal, as accounted for in J. Hagenauer, P. Hoeher, A Viterbi algorithm with soft decision outputs and its applications, IEEE, GLOBECOM 1989, Dallas, Tex., Nov. 1989, which is incorporated herein by reference is previously known.